The gamma ray pit at the University of Houston API Calibration Facilities defines the API unit for natural gamma ray logs used throughout the petroleum logging industry. The gamma ray pit includes stacked slabs of various porosity quarried limestone and an adjacent pit comprised of three 8-ft layers of cement having relatively low and high radioactivity levels. The facility was intended to provide calibration standards to unify log responses among service companies for neutron porosity and natural gamma ray logs. The pits were used to establish API units for both types of logs. For neutron logs, the API unit was defined as 1/1000th of the response in the 19 p.u. limestone formation. For gamma ray logs, the API unit was defined as 1/200th of the difference between responses from the high-activity and the lower-most low-activity zones.
The API unit definition for neutron porosity logs failed to survive the 1960s, and all modern neutron logs are now calibrated in porosity units. The API unit remains the industry standard for gamma ray logs and it is unlikely a newly defined unit will gain favor after almost 60 years of usage.
The API gamma ray pit, however, presents a number of challenges. When the pit was designed and built, modest-sized wireline tools were capable of operating in all of the known logging environments, and logging-while-drilling (“LWD”) instruments had not been discovered. Consequently, the design for the pit included a borehole that accommodated contemporary tools with little allowance for development of larger wireline tools or most LWD tools. The 4.9-in. inside diameter of the steel casing lining the pit barely accommodates tools recently developed for deep-water Gulf of Mexico exploration. Moreover, future wireline tools for high-pressure logging environments may also exceed the capabilities of the API facility.
In addition to its size limitations and the university's desire to repurpose the site, the API Calibration Facilities are rapidly deteriorating. Over the last decade, corrosion has completely destroyed the steel casing protruding from the pit at the surface. While the condition of the steel casing below the surface is unknown, it is probable that corrosive processes are also at work within the pit. Because the presence of the steel casing is an integral part of the API unit definition, subsurface deterioration of the casing is gradually altering the definition of the API unit and at some point the pit will become unusable. In addition, the facility's usefulness has diminished over the years in part because larger service companies have built their own tool response laboratories whose capabilities permit operations beyond those allowed at the API facility.
Thus, it is important to explore alternatives to preserve the essential features that form the basis of the definition for the API unit as an industry standard going forward. However, there is little interest among the major service companies to fund construction of a replacement facility or to attempt to relocate the existing pits.